/Engine/Plugins/Runtime/CommonUI/Source/CommonUI/Public/SCommonHierarchicalScrollBox.h
This is a near copy ofSScrollBox::OnNavigation. Ideally AppendFocusableChildren would be a virtual function in SScrollbox, but I don't have enough time to test functional changes to a core slate widget at the time of this writing. It was safer to isolate it. to this widget where I can test every case where it is used.
virtual [FNavigationReply](API\Runtime\SlateCore\Input\FNavigationReply) OnNavigation ( const [FGeometry](API\Runtime\SlateCore\Layout\FGeometry) & MyGeometry, const [FNavigationEvent](API\Runtime\SlateCore\Input\FNavigationEvent) & InNavigationEvent )